In many cases, an automatic paper supply apparatus used in an image reading apparatus is designed to supply an original to the image reading apparatus such that the side of the original which is to be read faces downwards, because of the manner of mounting of an optical image reading mechanism, such as a CCD, onto the body of the image reading apparatus. A bottom removal-type paper supply apparatus is used since it has the advantage that origins are stacked in the order in which the originals are placed.
A paper supply apparatus used in a typical image reading apparatus has a structure like that shown in FIG. 9. When there are a plurality of sheets of originals to be read, the plurality of sheets of paper 55 are stacked up, and they are set on a paper support base 56 so that the ends of the paper 55 ride atop a pickup roller 50. Due to the rotation of the pickup roller 50 so as to draw the paper 55 which was set into the apparatus, the bottom sheet of paper 55 is picked by the frictional force acting on the portion contacted by the pickup roller 50 and it is transported into the body of the apparatus. At this time, there are cases in which not just the bottom sheet but a number of sheets above it are also transported. However, the thickness of sheets which can pass is regulated by a paper supply gate 52, so the number of sheets which can be supplied to the apparatus is reduced, and only one sheet of paper is supplied into the apparatus by a separator roller 53 and a brake roller 54.
In this type of paper supply apparatus, in order to transport the paper 55 into the apparatus with certainty, it is necessary for the frictional force which acts in the contact region between the bottom sheet of the stacked paper 55 and the pickup roller 50 to be sufficiently larger than the frictional force acting between the paper 55 and the paper support base 56 or the frictional force acting between sheets of paper. The size of the frictional force acting in the contact region between the paper 55 and the pickup roller 50 is proportional to the coefficient of friction between the paper 55 and the pickup roller 50 and to the force with which the paper 55 is pressed against the pickup roller 50. Accordingly, in order to make the frictional force acting in the contact region between the paper 55 and the pickup roller 50 sufficiently large, it is effective to increase the coefficient of friction of the pickup roller 50 or to increase the force with which the paper 55 is pressed against the pickup roller 50.
If the force with which the paper 55 is pressed against the pickup roller 50 depends on the weight of the paper itself, when the number of stacked sheets is small, a sufficient frictional force can no longer be obtained, so a pressing roller 51 is provided which exerts a force pressing the paper towards the pickup roller 50. In order to increase the frictional force acting in the contact region between the paper 55 and the pickup roller 50, the pressure applied by the pressing roller 51 is increased, and thus, a sufficiently large frictional force is obtained.
However, if the pressure applied by the pressing roller 51 is made too large a value, the frictional force acting between sheets of paper also ends up being large, and there are cases in which it becomes the cause of paper misfeeds such as double feeds in which a plurality of sheets of paper are transported into the apparatus at the same time. Therefore, in a paper supply apparatus like that shown in FIG. 9, the pressure applied by the pressing roller 51 is restrained to at most a prescribed level at which double feeds and the like do not take place.
No problems occur when a small amount of paper 55 is stacked on the paper support base 56. However, when a large amount of paper 55 is stacked, the pressing force acting on the bottom sheet of paper 55 due to the weight of the stacked paper becomes large, and the frictional force acting between the paper 55 and the paper support base 56 and the frictional force acting between sheets of paper becomes too large, and there was the problem that the bottom sheet of paper 55 could not be transported into the apparatus given the frictional force acting in the contact region between the paper 55 and the pickup roller 50.
In order to solve the problem of the occurrence of misfeeds when a large amount of paper is stacked on a paper support base in this manner, as shown in FIG. 10, a paper supply apparatus which has a second pickup roller 57 which is a separate, additional pickup roller at a central portion of the paper support base 56 has been conceived (see, for example, Patent Document 1). In a paper supply apparatus having a second pickup roller 57 in this manner, when the amount of stacked paper becomes large, the force with which the bottom sheet of paper 55 is pressed against the second pickup roller 57 also increases due to the weight of the paper stacked on the bottom sheet of paper 55, so the frictional force acting in the contact region between the paper 55 and the second pickup roller 57 also increases. As a result, when the second pickup roller 57 is rotatably driven, a force acts so as to convey the paper 55 into the apparatus. This assists the conveying force acting between the paper 55 and the pickup roller 50, and the paper 55 can be transported into the apparatus.
However, in a paper supply apparatus having such a second pickup roller, although it works effectively when a large amount of paper is stacked as shown in FIG. 11(B), when the stacked amount is small, due to a difference between the coefficients of friction of the first pickup roller and the second pickup roller and other factors, a difference in transportation amount is produced between the first pickup roller and the second pickup roller, and in some cases, as shown in FIG. 11(A), the paper ends up bending and paper misfeeds occur.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2002-308448